The implementation of present day video compression standards, such as MPEG, H.263 and JVT (also known as ISO/ITU H.264) result in the division of video frames into macroblocks and slices. A slice generally contains a given number of consecutive macroblocks. However, the use of the Flexible Macroblock Ordering option for video compressed in accordance with the JVT compression standard can result in the macroblocks in a slice are not necessary being adjacent to one another in display order. Variable Length Coding (VLC) techniques can enhance compression efficiency, at the expense of error resiliency. An error in a bit stream can propagate undetected and corrupt all subsequent macroblocks in a slice.
To enable error recovery, the header of each slice contains a resynchronization marker. Such markers constitute the earliest point at which proper decoding can take place following a bit error. Slice headers begin on a byte boundary so as to be byte aligned whereas individual macroblocks are not byte aligned. When using Context-based Adaptive Binary Arithmetic Coding in the connection with the coding of a video bit stream in accordance with the JVT compression standard, multiple symbols can be coded as a single bit, so that macroblock boundaries are not even bit aligned. In this regard, precise identification of when bit errors occur the decoder becomes difficult to variable length code words. Most bit patterns in a corrupted bit stream represent valid code words, even if they don't represent correct values. In some cases, identification of invalid code words can occur if the decoded value reaches beyond a legitimate range, but this will not always become apparent until incorrect decoding of a variable, unknowable amount. Thus all macroblocks in a slice following an error remain suspect for corruption.
Video error detection can occur by decoding a video frame and then checking the frame for spatial discontinuities at macroblock and slice boundaries, in the pixel domain. Various proposals now exist for video error detection and concealment. Follow detection of errors in particular macroblocks, error concealment can occur on the corrupt macroblocks. Concealed macroblocks will generally appear perceptually better than unconcealed corrupt macroblocks, but perceptually worse than correctly decoded macroblocks. However, present day concealment algorithms consume significant resources, making then computationally expensive. Moreover, such algorithms are also prone to false positives (concealing correctly decoded macroblocks) and false negatives (failure to conceal corrupted macroblocks.)
One approach to overcoming the problem of corrupted macroblocks recommends transmitting an entire slice in a single IP packet. Should a slice become lost during transmission, all macroblocks in the slice become marked as missing and thus require concealment. As previously mentioned, all data in a packet with a known transmission error will be marked as corrupted, discarded and concealed. Thus, an entire slice will now become lost in the case of a detected transmission error. Assigning relatively few macroblocks per slice will improve error resilience, but at a significant cost of coding and transport efficiency.
Thus, a need exists for a technique for detecting bit stream errors enjoys a relatively low computational complexity.